In general, a down-draw process and an up-draw process are known as methods for continuously forming, from molten glass, a sheet glass used in the manufacture of a flat display or the like such as by liquid crystal display. The down-draw process is basically classified into an overflow down-draw process and a slot down-draw process.
As an example, means for manufacturing sheet glass by the overflow down-draw process will be described. The temperature of molten glass obtained by heating a glass raw material in a melting furnace is reduced until the molten glass has a viscosity suitable for the formation. Thereafter, the molten glass is flown down in a supplying tube 62 shown in FIG. 4a provided at a downstream end portion of a supplying passage, and is continuously supplied from one side of a forming vessel (a groove-shaped overflow vessel) 11a formed on an upper portion of a formed body 11b having a cross-section of substantially wedge in the forming furnace 11. In general, a vessel (pot) is provided at an upper end of the supplying tube 62 such that the molten glass is subjected to soaking near the temperature suitable for the forming. Moreover, in the forming furnace 11, the molten glass G flown over the overflow vessel 11a flows down along both side surfaces 11ba of the formed body 11b to be fused at a lowest end 11bc, and the fused molten glass is drawn downwardly so that a sheet glass 13 is continuously formed, as shown in FIG. 4b. 
However, such conventional method has led to the following problems. The molten glass flowing out of the melting furnace gradually reduces temperatures while moving downstream of the supplying passage, and is subjected to a temperature control to be supplied to the forming furnace when reaching the temperature suitable for the forming. In addition, since a viscosity of the molten glass increases as the temperature decreases, the flow of molten glass increases resistance as the molten glass moves to the downstream of the supplying passage. Therefore, as shown in FIG. 4a, when the supplying tube 62 is directly coupled to the forming furnace 11 (overflow vessel 11a) and has a constant diameter (flow passage area), a supplying amount of the molten glass to the forming furnace 11 is controlled by a resistance applied to the molten glass from a portion provided at the extreme downstream end of the supplying tube 62 near the forming furnace 11. For this reason, when a thickness distribution or the like of the sheet glass 13 is changed, the temperature of the portion of the supplying tube 62 near the forming furnace 11 must be changed so that the viscosity is adjusted to an appropriate value. However, in the conventional method, since the entire flow of the molten glass existing in the long passage of the supplying tube 62 is affected by the above temperature change, the flow rate of the molten glass supplied to the forming furnace 11 is likely to be changed rapidly and a relatively long time is required for reaching the steady state of the flow rate during which a good quality of sheet glass cannot be obtained.
As a structure addressing the problems, it has been known that the supplying tube is divided into a small diameter tube provided at the upstream side and a large diameter tube provided at the downstream side, a lower end of the small diameter tube is inserted into an upper end of the large diameter tube, and the molten glass flowing into the large diameter tube from the small diameter tube is supplied to the forming vessel in the forming furnace (see e.g., Patent Documents 1 and 2 listed below). More specifically, as schematically shown in FIG. 5, a supplying tube 72, which is provided at a downstream end portion of the supplying passage of the molten glass from the melting furnace to a forming furnace 21, is composed of a small diameter tube 72a of which upstream end is connected to, for example, a vessel for soaking the molten glass, and a large diameter tube 72c of which downstream end 72ca is connected to one side of an overflow vessel 21a in the forming furnace 21, and the downstream end portion of the small diameter tube 72a is inserted into the inside of the upstream end portion of the large diameter tube 72c. According to such structure, even when the temperature near the downstream end 72ca of the large diameter tube 72c is changed during the changing of thickness distribution or the like of a sheet glass 23, a fluid level L of the molten glass at the upstream end portion of the large diameter tube 72c merely moves up and down and the flow of the molten glass in the small diameter tube 72a is little affected. Therefore, the flow rate of the molten glass is prevented from changing rapidly. As a result, it is possible to change thickness distribution or the like of the sheet glass 23 while maintaining a stable flow rate.
[Patent Document 1] JP No. 2001-80922A
[Patent Document 2] United States Patent Application Publication No. 2003/0110804